The present invention generally relates to power conversion systems used in AC propulsion or traction motors and, more particularly, to an improved method for control of such systems so as to reduce audible monotonic noise which may be produced during motoring and electrical braking.
Kumar, U.S. Pat. No. 5,396,214, describes a power conversion system including electric power converters, such as inverters using any of various well-known pulse width modulation ("PWM") techniques, to control the motors during a motoring mode of operation and dynamic braking circuits including other converters, such as DC choppers which may use similar PWM techniques as used in the inverters, to control power conversion during an electrical braking mode of operation. During motoring, the traction motors and inverters may produce audible noise having a level and frequency generally determined by many factors including the value of the PWM switching frequency of inverter operation. The noise, especially monotonic noise, that is, noise primarily containing a generally steady energy level in a relatively narrow frequency band, can be irritating to the average listener. Similarly, during electrical braking, the traction motors, the inverters and the dynamic braking circuits may produce audible noise resembling the type of noise produced during motoring.
The foregoing noise condition can become even more troublesome when multiple converters, such as the inverters coupled to respective sets of the traction motors, or the DC choppers in the dynamic braking circuits, use PWM signals having the same frequency, as the combined effect of such operation can exacerbate an already annoying condition.
Several approaches for reducing these noise levels have been proposed heretofore but each such approach suffers from significant drawbacks. These approaches include adding sound reduction barriers around the propulsion equipment, changing the motor designs, controlling the resonance frequencies of the system, and achieving higher PWM switching frequencies. These approaches, unfortunately, are expensive to implement, particularly if the equipment is already designed and built, and generally such approaches introduce additional equipment weight due to added material and/or components. Additional weight leads to reduced available power for transporting cargo and/or passengers.